Generally speaking, backlight modules of various structures are used to improve the brightness of the backside illumination flat-panel displays such as the liquid crystal displays (LCDs) including not only electronic illumination boards, notebook computer displays, word processors, desktop computer displays, television sets, camcorders, but also displays of vehicles and aircrafts. The backlight module is provided with a light source, a light guide plate and a plurality of optical sheets, and light from the light source is emergent to the display after passing through the light guide plate and the plurality of optical sheets.
Recently, optical sheets of various forms are used in the backlight modules in order to improve characteristics of the brightness and the view angle of the image display device like the LCD. Among the above optical sheets, optical sheets of various forms are used to improve the light characteristics, and they are classified into condensing optical sheets and diffusing optical sheets. When the condensing optical sheets are used, the brightness characteristic varies largely with the view angle although the brightness is improved, and the view angle characteristic is degraded. If the diffusing optical sheets are used, the brightness is decreased although the view angle characteristic is improved.
Thus, various researches and developments are made for an optical compound sheet having both the light-condensing function and the light-diffusing function.
FIG. 1 is a schematic diagram of an existing backlight module. As shown in FIG. 1, the backlight module includes a light source 4; a light guiding plate 5 for changing the path of light emergent from the light source 4 to guide the light to a display portion (not shown); a plurality of optical sheets 6, 7, 8 disposed between the light guiding plate 5 and the display portion (not shown) to improve the efficiency of the light emergent from the light guiding plate 5; a reflecting plate 2 disposed below the light guiding plate 5 in order to prevent the light generated by the light source 4 from being exposed to the external; a light source reflecting plate 3; and a modular frame 1 on which the reflecting plate 2, the light source reflecting plate 3, the light guiding plate 5 and the optical sheets 6, 7, 8 are laminated in sequence.
During the manufacturing, the optical sheets 6, 7, 8 may be formed by a diffusion sheet 6, a prism sheet 7 and a protection sheet 8. In order to improve the light condensing efficiency, a structured pattern 9 of the prism sheet 7 has a triangular cross-section, and the shape of a tip of the structured pattern 9 which forms vertex angle is changed variously, wherein the structured pattern 9 having a vertex angle of 90° is acknowledged as providing the best brightness. The function of the diffusion sheet 6 is to diffuse the light emergent from the light guiding plate 5 and incident to the display portion (not shown), so that the brightness distribution of the light becomes uniform.
In order to have both the light-condensing function and the light-diffusing function, the backlight module uses the optical compound sheet to manufacture an integrated optical sheet including minute structured patterns formed on the single base layer and diffusion patterns formed below the base layer.
The optical compound sheet may be disposed on the light guiding plate 5 or attached onto the prism sheet 7 depending on the configuration required by the backlight module.
FIG. 2 is a cross-section diagram of one embodiment of an existing optical compound sheet. Generally, an optical compound sheet 10 includes triangular structured patterns 12 formed on a base layer 11 for light condensation and beads 14 formed below the base layer 11 for light diffusion.
Thus, the beads 14 formed below the base layer 11 have the function of light diffusion before converging external incident light to the triangular structured pattern 12. For example, when the optical compound sheet 10 is applied to the backlight module, light emergent from the light guiding plate 5 is diffused by the beads 14 instead of any other diffusion sheet 6.
The beads 14 may be disposed between the triangular structured pattern 12 and the base layer 11. In addition to the beads 14, micro-particles of 2˜5 μm may also additionally be used to realize the diffusing function.
However, the particles of the existing optical compound sheet are larger relative to the structured pattern which usually has a predetermined interval of 50 μm and a height of 25 μm, so it is difficult to uniformly arrange the particles when they are embedded into the structured pattern. In addition, when a structural deformation is tried, the whole brightness of the light passing the optical compound sheet is decreased due to the haze increment and the Total Transmission (TT) loss.
In addition, since a structured pattern layer or the beads have a weak anti-scratching performance, a surface damage will easily occur during the manufacturing, laminated assembly or transportation. Thus, there are many surface flaws and the product yield is low. That is to say, because the micro-particles and the beads below the base layer form protruded structures, in sizes of 2˜5 μm, scratches may be easily caused during the assembly and manufacturing of the optical sheets, and a brightness loss of about 2% is resulted due to the defects of the sizes and the protruded structures.
In order to solve such problem, some patents intended to improve the anti-scratching performance have been disclosed.
U.S. Pat. No. 7,269,328 discloses an optical prism film for improving brightness, and the patent forms enhancement layers of different hardness coefficients below the base layer in order to prevent the damage caused by scratches. However, such prior art is only suitable to the common backlight module rather than the optical compound sheet, because both sides of the base layer of the optical compound sheet have functional coatings, the functionality of original coatings will be damaged if the enhancement layers are added.
American patent US2007/0121227A1 discloses a technique using scattering particles of silicon resin inside the structured pattern, in which the scattering particles are distributed in sizes of 0.5-30 μm. Further, in the technique disclosed by Korean patent registered as No. 10-0636739, transparent nano-particles are used in the ultra violet (UV) cured resin layer forming the minute structured pattern, so as to improve the hardness.
However, the above two prior arts are only limited to the optical prism sheet or the diffusion sheet, and they are difficult to be applied to the optical compound sheet having both the light-condensing function and the light-diffusing function. In addition, during the formation of the structured pattern, the surface of the structured pattern may not be smooth due to the existence of internal scattering particles, and the other side of the structure pattern is easier to be scratched and deficient.